Class Nine Science NCERT Solutions Chapter 5

Chapter 5: The Fundamental Unit of Life

Chapter 5 : The Fundamental Unit of Life

Intext Questions (Page No. 59)

1. Who discovered cells and how?

Ans. Robert Hooke, by observing a thin section of cork under the self-designed microscope.

2. Why is the cell called a structural and functional unit of life?

Ans. All living organisms are made up of cells and these cells perform all the functions essential for the survival of the organisms, e.g., respiration, digestion, excretion, etc. Thus, the cell is the functional unit of life.

In a unicellular organism, a single cell carries out all these functions and in multicellular organisms, different groups of cells carry out different functions. Thus, the cell is the structural unit of all living organisms.

Intext Questions (Page No. 61)

1. How do substances like CO2 and water move in and out of the cell? Discuss.

Ans. When the concentration of carbon dioxide is more inside the cell than outside, CO2 diffuses from the cell to outside of the cell. If CO2 concentration inside the cell is less, CO2 moves inside the cell from outside.

The water moves in and out of the cell by the process of osmosis. Osmosis is the passage of water from a region of high water concentration to a region of low concentration of water through a semi-permeable membrane.

2. Why is the plasma membrane called a selectively permeable membrane?

Ans. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane.

Intext Questions (Page No. 63)

1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.


Ans. 2. Nuclear region: An undefined nuclear region and the nuclear membrane is absent and known as the nucleoid.

4. The nuclear membrane as well as membrane-enclosed organelles present.

Intext Questions (Page No. 65)

1. Can you name the two organelles we have studied that contain their own genetic material?

Ans. Mitochondria and plastids.

2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

Ans. The damaged cell will not be able to perform functions like respiration, obtaining nutrition and cleaning of waste materials or forming new proteins.

3. Why are lysosomes known as suicidal bags?

Ans. Lysosomes are called suicidal bags because they secrete certain enzymes, which helps in digesting the dead cell organelles and sometimes they also cause the digestion of the whole cell.

4. Where are proteins synthesised inside the cell?

Ans. Ribosomes are the sites of protein synthesis.


1. Make a comparison and write down ways in which plant cells are different from animal cells.


2. How is prokaryotic are different from a eukaryotic cell?


3. What would happen if the plasma membrane ruptures or breaks down?

Ans. If the plasma membrane ruptures or breaks down, the constant internal composition of the cell will be lost and it will not be able to perform basic functions.

4. What would happen to the life of a cell if there was no Golgi apparatus?

Ans. The material synthesised in the ER will not be packed, stored and transported to various targets and lysosome formation will also not take place.

5. Which organelle is known as the powerhouse of the cell? Why?

Ans. Mitochondria are known as the powerhouse of the cell because it releases the energy required for different activities needed for life.

6. Where do the lipids and proteins constituting the cell membrane get synthesised?

Ans. Proteins in RER and lipids in SER.

7. How does an Amoeba obtain its food?

Ans. Amoeba acquires its food through the process of endocytosis. This process takes place due to the flexible nature of the cell membrane which forms the structure pseudopodia of Amoeba. The flexible pseudopodia enable the Amoeba to engulf food and other material from its external environment.

8. What is osmosis?

Ans. Osmosis is the movement of water from a region of higher concentration to a region of lower concentration through a cell membrane or outer semi-permeable membrane until an equilibrium is reached.

9. Carry out the following osmosis experiment: Take four peeled potato halves and scoop each one out to make potato cups one of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water.

(a) Keep cup A empty

(b) Put one teaspoon sugar in cup B

(c) Put one teaspoon salt in cup C

(d) Put one teaspoon sugar in the boiled potato cup D.

Keep these for two hours. Then observe the four potato cups and answer the following :

(i) Explain why water gathers in the hollowed portion of B and C

(ii) Why is potato A necessary for this experiment?

(iii) Explain why water does not gather in the hollowed-out portions of A and D.

Ans. (i) When unboiled potato cups B and C were put into a trough containing water, the cells of potatoes gained water by endosmosis.

When a teaspoon of sugar and salt were added later in the hollowed portions B and C cups respectively, water movement occurred through the plasma membrane of the cells from within the cells of potato into the hollowed portion of both B and C cups because of exosmosis

(ii) Potato A is necessary for this experiment for comparison, it acts as a reference.

(iii) Water does not gather in the hollowed portion of potato cup A because it does not possess higher osmotic concentration on than the cells of potato.

Potato cup D is boiled potato cup on boiling, potato cells die and the membrane of the potato

cells lose their permeability. The result is, when a teaspoon of sugar is added into the hollowed portion of boiled potato cup D, water does not come out from within the potato cells into the hollowed portion.